The data for the study on the epidemiology of bovine mastitis were collected fromNovember 2015 to February 2016 to investigate the prevalence of mastitis in administrative towns of three selected districts of Arsi zone, Oromia Regional State, namely Iteya, Sagure, and Asella, Ethiopia; and to analyze the association between the risk factors and mastitis occurrence. Sample cows were selected and examined for clinical mastitis and at the same time, milk samples were evaluated both clinically and using the California Mastitis Test (CMT). Some parameters of the cows which have biological relation with this disease together with other potential risk factors were collected along with primary data collection. The analysis of the primary data showed that overall mastitis prevalence in the locations covered by the study was 28.6% of which 83.3% was subclinical, while 16.7% were clinical mastitis cases. Analysis related to the investigation of the existence of an association between risk factors and mastitis revealed that there was a significant association between the occurrence of mastitis and two of the risk factors evaluated, breed and age of the cow. The other risk factors investigated (i.e. parity, location, and lactation stage had no significant association with mastitis occurrence). Since mastitis is a complex disease that results from the interaction of different factors, it may be difficult to limit it to only some given factors. Environment and management factors and their combination may also have an additional impact on the epidemiology of the disease. A prevalence with the indicated magnitude together with the added effect of a lower level of awareness of the society about milk-associated zoonoses implies that this disease causes significant public health problems. In addition, mastitis may cause silent economic loss through unnoticed production loss along with reduced milk yield from subclinical and clinical (e.g. blind teats) cases. Due to a lack of awareness about mastitis and/or negligence, farmers in all corners of the country are suffering from economic and public health problems imposed by mastitis. This lack of awareness about mastitis-associated problems and/or negligence could be a possible reason for the use of raw milk which is a common scenario particularly in pastoral areas and the selling of adulterated milk which is becoming increasingly practiced in Ethiopia which is also true in these specific localities.